Approach flow system

ABSTRACT

The invention relates to an arrangement for feeding a fibrous suspension ( 1 ) to at least one headbox ( 8 ) of a paper or cardboard machine with at least one diluting device ( 5 ), in which the fibrous suspension ( 1 ) is produced by mixing a high-consistency suspension ( 6 ) with a diluting liquid, wherein the diluting liquid is formed at least in part by backwater ( 2 ) of the paper or cardboard machine, the backwater ( 2 ) is pre-degassed in at least one open channel ( 3 ) and subsequently is guided into a diluting device ( 5 ) via a degassing device ( 4 ). The expenditure is to be reduced thereby in that the potential energy of the fibrous suspension ( 1 ) in the channel ( 3 ) is not above the potential energy of the fibrous suspension ( 1 ) in the diluting device ( 5 ) and the pressure at the outflow of the degassing device ( 4 ) is between −0.3 and 1 bar or that the potential energy of the fibrous suspension ( 1 ) in the channel ( 3 ) is above the potential energy of the fibrous suspension ( 1 ) in the diluting device ( 5 ).

The invention relates to an arrangement for feeding a fibrous suspensionto at least one headbox of a paper or cardboard machine with at leastone diluting device, in which the fibrous suspension is produced bymixing a high-consistency suspension with a diluting liquid, wherein thediluting liquid is formed at least in part by backwater of the paper orcardboard machine, the backwater is pre-degassed in at least one openchannel and subsequently is guided into a diluting device via adegassing device.

Methods of this type are used to supply paper or cardboard machines withfibrous suspension. The stock feeding systems required thereto are knownin principle. The fibrous suspension to be supplied receives the majorpart of the fibers from a high-consistency suspension provided in thestock preparation unit. The high-consistency suspension has, e.g., atypical consistency of between 2.5 and 5%. By the addition of a dilutingliquid, in particular backwater of the paper machine, the consistency islowered to a value that is favorable for operating the headbox of thepaper machine. Although the backwater is optimally suited for thisdiluting task, there are, however, problems due to the high gas content,the predominant part being air.

The largest proportion of these gases escapes very quickly, however, theresidual gases often have to be removed in a complex manner. Otherwise,the quality of the paper produced would be unduly lowered.

Known solutions are large degassing containers in which a permanentnegative pressure is maintained by evacuation, which negative pressurecorresponds to the steam pressure of the suspension to be degassed.

In DE 10 2004 051 327 an arrangement is therefore proposed that issimilar to the preamble of claim 1.

The object of the invention is to reduce the expenditure in arrangementsof this kind.

According to the invention, the object was attained in that thepotential energy of the fibrous suspension in the channel is not abovethe potential energy of the fibrous suspension in the diluting deviceand the pressure at the outflow of the degassing device is between −0.3and 1 bar or that the potential energy of the fibrous suspension in thechannel is above the potential energy of the fibrous suspension in thediluting device.

If the potential energy of the fibrous suspension in the channel ishigher than in the diluting device, a pump between the assemblies can beomitted, which considerably reduces the expenditure. The difference ofthe potential energy thereby essentially determines the throughput offibrous suspension. If the difference of the potential energy is toogreat, an overpressure must be reduced via throttles or the degassingdevice, which is not advantageous in terms of energy.

The difference of the potential energy of the fibrous suspension in thechannel and the diluting device should therefore correspond to no morethan an equivalent of 8 m difference in height.

If the potential energy of the fibrous suspension in the channel is notabove that in the diluting device, the degassing device must be arrangedand/or designed such that at the outflow thereof a pressure between −0.3and 1 bar is adjusted.

The height difference between the channel and the degassing device, butalso the pressure build-up by the degassing device due to a pump actionof the same have a decisive influence.

If a pressure of less than −0.3 bar is adjusted at the outflow of thedegassing device, a small pump should be utilized between the degassingdevice and the diluting device.

Regardless of the embodiment, the pressure at the outflow of thedegassing device should advantageously be higher than the pressure atthe inflow of the diluting device, so that the pressure differenceensures the transport of the fibrous suspension between these elements.

To this end it is advantageous if the outflow of the degassing device isarranged above the inflow of the backwater into the diluting device. Inthis manner the flow of the backwater from the degassing device to thediluting device can take place entirely without conveyor elements.

In any case, the pressure at the outflow of the degassing device shouldbe no more than 0.5 bar less than the pressure at the inflow of thediluting device. In this case, a small pump between the degassing deviceand the diluting device can take over the transport of the fibroussuspension.

For adjustment to the conditions of the installation, but also foroptimization, the flow of the fibrous suspension between the channel andthe degassing device and/or between the degassing device and thediluting device should preferably be restricted in a changeable manner.

Moreover, it is advantageous if the inflow of the degassing device isarranged above the channel.

In the interest of a degassing that is as efficient as possible, thedegassing device should be connected to a vacuum source. Degassingdevices with a rotating rotor are particularly suitable, in the interiorof which the backwater is guided, wherein the gas collecting in thecenter due to centrifugal forces is suctioned by the vacuum source.

The diluting device can be embodied as a closed or as an open container.

After the diluting device, the fibrous suspension should be guided tothe headbox via a stock pump if possible in a closed system.

In the event of the standstill of the degassing device, the liquid levelin the degassing device falls to that of the diluting device. In orderto generate the difference in level necessary for the transport of thebackwater at the start of the degassing device, the stock pump should bestarted before start-up of the degassing device.

The method is to be used particularly favorably with paper machines, thescreening rate of which is between 800 and 1600 m/min, since thequantity of the air contained in the backwater depends considerably onthe screening rate.

The invention is explained in more detail below based on an exemplaryembodiment. In the attached drawing, the figure shows a layout plan.

A fibrous suspension 1 is thereby generated in the usual manner bymixing a high-consistency suspension 6 with a diluting liquid andthereafter has essentially the desired consistency at this point for theoperation of the headbox 8 of the paper or cardboard machine. As isknown, headbox consistencies are in the range between 0.5 and 2%,generally around 1%.

The diluting liquid, which is fed in a diluting device 5, in the exampleshown here comes from the first backwater 2, that is, the wateraccumulated in the forming region 10 of the paper or cardboard machine.It is often referred to as backwater I, as opposed to the secondbackwater II accumulating on the paper machine later, which secondbackwater contains far fewer fines.

It is known that backwater 2 of this type is mixed among other thingswith a considerable proportion of air and possibly other gases. Thebackwater 2 is collected below the paper machine screen and drained offat the side. A large part of the air contained can already escape in theopen channels 3 used for this. However, it is often expedient toeliminate a further gas proportion from the backwater 2 in a subsequentdegassing device 4, as is described, for example, in DE 10 2004 051 327.

The degassing device 4 is constructed as a cylindrical or conicalcontainer and provided according to the invention with a rotor 13. Thefibrous suspension 1 reaches the essentially cylindrical interior 14 ofthe rotor 13 and is put into rapid rotation. This principle correspondsto that of a solid bowl centrifuge. As a result of the centrifugalforces, the gases contained move inward and are suctioned off from thecenter by a vacuum source 7.

The centrifugal forces can be at least 5 times, preferably 10 times theacceleration of gravity. The vacuum source 7 requires no negativepressure that corresponds to or is similar to the steam pressure of thesuspension. Values customary for the negative pressure here areapproximately 0.8 to 0.9 bar.

The inlet 16 of the degassing device 4 can be arranged centrally oreccentrically. The liquid added through it is first greatly acceleratedin the circumferential direction, for which, e.g., acceleration ribs 24can be used. As a result of the centrifugal forces, the liquid bearsagainst the interior wall of the rotor 13, wherein the gas containedtherein, in particular the air, moves towards the center of the rotor13. As a rule, a discontinuity surface forms between the liquid and thegas.

The gas can be suctioned off through a central degassing tube with theaid of the vacuum source 7.

In the area of the outflow 15, the wall of the rotor 13 is therebyembodied to be permeable to liquids, e.g., by placing openings in thisarea.

Advantageously, the outflow 15 is mounted tangentially such that therotational flow of the degassed liquid causes a pressure build-up in theoutflow 15. The openings in the wall of the rotor 13 are so big thatthey do not lead to a screening of the degassed liquid.

The degassing device 4 in addition can also be provided with an outerscreen basket, via which the backwater 2 pressed radially outwards canbe divided into accepts and rejects.

The degassed backwater 2 is guided via the outflow 15 of the degassingdevice 4 directly into the diluting device 5, which here is embodied asan open container. The degassing can be carried out more easily due tothe far lower proportion of solids of the backwater 2 compared to thefibrous suspension 1. However, this requires at most a merely low gascontent in the high-consistency suspension 6 added to the dilutingdevice 5 in order to avoid losses in quality.

A mixing device described in DE 100 50 109 is particularly suitable as adiluting device 5. The important aspect here is that the intermixing ofthe flows takes place via an injection of the high-consistencysuspension 6 into the backwater 2, wherein the inflow speed of thehigh-consistency suspension 6 is preferably 3 times to 15 times the flowspeed of the backwater 2.

As is known, there are many paper or cardboard machines in which theheadbox 8 is supplied not only with the fibrous suspension 1 alreadymentioned, but in addition with a diluting liquid which is added in ametered manner at various points seen over the width of the headbox 1.It is thus possible to influence, in particular optimize, e.g., thecross profile of the fibrous layer formed with the headbox 8.

The figure shows an option of this type, in which the backwater 2 isused not only as a diluting liquid for the high-consistency suspension6. A part of the backwater 2 deaerated in the degassing device 4 isconveyed with the diluting water pump 20 as diluting liquid into ametering device 17 of the headbox 8, in which it is divided and thefibrous suspension 1 is added at different points.

In a paper mill there are often further points at which further dilutingliquid deaerated by the method can be used.

A last screening stage 18 is often placed upstream of the headbox 8 whensupplying a paper machine, in order to prevent contaminants from gettinginto the headbox which interfere with the operation of the headbox 8 orthe paper machine.

This screening function can also be carried out in combination with thedegassing function, as described above, in the same device.

After leaving the diluting device 5, the fibrous suspension 1 is guidedvia a stock pump 9 in a closed system (i.e., without any open containersor vats) via a cleaner unit 11, a further pump 12 and the screening step18 to the headbox 8.

The potential energy of the fibrous suspension 1 in the channel 3 ishere higher than the potential energy of the fibrous suspension 1 in thediluting device 5. As a result, pumps for conveying the backwater 2 canbe omitted in this area, which has a considerable effect on costs.However, the energy difference should not be too great either, since theexcess energy otherwise has to be reduced via adjustable throttleelements 21, 22 in the form of valves.

In order to be able to ensure a transport of the fibrous suspension 1between the degassing device 4 and the diluting device 5 also without anadditional pump, the pressure at the outflow 15 of the degassing device4 is higher than the pressure at the inflow 19 of the diluting device 5.Moreover, the outflow 15 of the degassing device 4 is arranged above theinflow 19 of the backwater 2 in the diluting device 5.

At rest the liquid level in the degassing device 4 falls to the liquidlevel in the diluting device 5, which is why the level differencenecessary for the transport of the backwater 2 must be reestablishedbefore starting up the degassing device 4.

This can be achieved in that the liquid level in the diluting device 5is reduced to the necessary amount with the stock pump 9 before startingup the degassing device 4.

1. An arrangement for feeding a fibrous suspension (1) to at least oneheadbox (8) of a paper or cardboard machine with at least one dilutingdevice (5), in which the fibrous suspension (1) is produced by mixing ahigh-consistency suspension (6) with a diluting liquid, wherein thediluting liquid is formed at least in part by backwater (2) of the paperor cardboard machine, the backwater (2) is pre-degassed in at least oneopen channel (3) and subsequently is guided into a diluting device (5)via a degassing device (4), characterized in that the potential energyof the fibrous suspension (1) in the channel (3) is not above thepotential energy of the fibrous suspension (1) in the diluting device(5) and the pressure at the outflow of the degassing device (4) isbetween −0.3 and 1 bar.
 2. An arrangement for feeding a fibroussuspension (1) to at least one headbox (8) of a paper or cardboardmachine with at least one diluting device (5), in which the fibroussuspension (1) is produced by mixing a high-consistency suspension (6)with a diluting liquid, wherein the diluting liquid is formed at leastin part by backwater (2) of the paper or cardboard machine, thebackwater (2) is pre-degassed in at least one open channel (3) andsubsequently is guided into a diluting device (5) via a degassing device(4), characterized in that the potential energy of the fibroussuspension (1) in the channel (3) is above the potential energy of thefibrous suspension (1) in the diluting device (5).
 3. The arrangementaccording to claim 2, characterized in that the difference of thepotential energy of the fibrous suspension (1) in the channel (3) andthe diluting device (5) corresponds to no more than an equivalent of 8 mdifference in height.
 4. The arrangement according to claim 1,characterized in that the pressure at the outflow of the degassingdevice (4) is higher than the pressure at the inflow (19) of thediluting device (5).
 5. The arrangement according to claim 1,characterized in that the pressure at the outflow (15) of the degassingdevice (4) is no more than 0.5 bar less than the pressure at the inflow(19) of the diluting device (5) and a pump is arranged between thedegassing device (4) and the diluting device (5).
 6. The arrangementaccording to claim 1, characterized in that the fibrous suspension flow(1) between the channel (3) and the degassing device (4) is preferablyrestricted in a changeable manner.
 7. The arrangement according to claim1, characterized in that the fibrous suspension flow (1) between thedegassing device (4) and the diluting device (5) is preferablyrestricted in a changeable manner.
 8. The arrangement according to claim1, characterized in that the inflow (16) of the degassing device (4) isarranged above the channel (3).
 9. The arrangement according to claim 1,characterized in that the outflow (15) of the degassing device (4) isarranged above the inflow (19) of the backwater (2) into the dilutingdevice (5).
 10. The arrangement according to claim 1, characterized inthat the degassing device (4) is connected to a vacuum source (7). 11.The arrangement according to claim 10, characterized in that thedegassing device (4) has a rotating rotor (13), in the interior of whichthe backwater (2) is guided, wherein the gas collecting in the centerdue to centrifugal forces is suctioned by the vacuum source (7).
 12. Thearrangement according to claim 1, characterized in that the dilutingdevice (5) is embodied as a closed container.
 13. The arrangementaccording to claim 1, characterized in that the diluting device (5) isembodied as an open container.
 14. The arrangement according to claim 1,characterized in that after the diluting device (5), the fibroussuspension (1) is guided via a stock pump (9).
 15. The arrangementaccording to claim 14, characterized in that the stock pump (9) guidesthe fibrous suspension (1) to the headbox 8 in a closed system.
 16. Amethod for starting up an arrangement according to claim 14,characterized in that the stock pump (9) is started before start-up ofthe degassing device (4).